1. Field of the Invention
The present invention relates to a vehicle position recognition apparatus that senses a magnetic field formed on a road on which the vehicle is traveling by vehicle-mounted magnetic field sensors to recognize the self-position (e.g., position of itself) of the vehicle, and more particularly to an apparatus for judging whether the strength of a detected magnetic field is suitable for the position recognition of a vehicle.
2. Description of the Related Art
FIG. 14 is a diagram showing the installed state of a magnetic field induction cable that is employed in a vehicle position detector disclosed, for example, in Japanese Patent Laid-Open No. 8-16236. The magnetic field induction cable 14-1 is offset alternately by D/2 at intervals of a predetermined distance L with respect to a target induction line (indicated by a broken line) provided on a road 14-0, and the induction cable 14-1 is installed continuously in a square wave path.
The aforementioned D represents an arbitrary distance which satisfies D&lt;W (where W is the distance between left and right magnetic field sensors mounted in a vehicle). The resultant magnetic field (e.g., first resultant magnetic field) Z of the detected magnetic field strengths VR and VL of the right and left magnetic field sensors is computed by the following Eq. (1). EQU Z=(VL-VR)/(VL+VR) (1)
Furthermore, a second resultant magnetic field Zc is computed by the sum of the magnetic field Za at the a-point of the magnetic field induction cable 14-1 and the magnetic field Zb at the b-point. EQU Zc=Za+Zb (2)
The second resultant magnetic field Zc and the lateral displacement quantity X (e.g., quantity that a vehicle is displaced laterally from a target induction line) correspond to 1:1 at an interval of X-(W+D)/2, (W+D)/2!, and the lateral displacement quantity X is determined.
Now, the operation of a conventional apparatus will be described in reference to FIGS. 15 and 16.
FIG. 15 is a block diagram of the conventional apparatus. The vehicle is provided with a right magnetic field sensor 15-1 and a left magnetic field sensor 15-3, which are laterally spaced by distance W. The magnetic field strengths VR and VL detected by the magnetic field sensors 15-1 and 15-2 are inputted to a CPU 15-2 and the resultant magnetic field Zc of the strengths VR and VL is computed.
Also, the relation between the second resultant magnetic field Zc and the lateral displacement quantity X is previously computed and the relation is stored on memory 15-4 as a map.
The CPU 15-2 accesses the memory 15-4 to read out the lateral displacement quantity X corresponding to the computed second resultant magnetic field Zc. The lateral displacement quantity X is output as a control signal to a steering actuator (not shown) to perform a steering operation so that the lateral displacement X is corrected, and a vehicle is automatically controlled.
Next, the computation processes by the CPU 15-2 will be described with a flowchart of FIG. 16. The CPU 15-2 reads a value C of a wheel pulse counter (not shown) (step 16-1) and computes a traveled distance by multiplying the counter value C by a traveled distance d per one pulse.
Then, it is judged whether the traveled distance exceeds L (in FIG. 14 the length of the left or right offset portion) (step 16-2). When it has exceeded L, the counter value is reset (step 16-3) and the detected magnetic field strengths VR and VL are read from the right and left magnetic field sensors 14a and 14b (step 16-4 and step 16-5).
The CPU 15-2 computes the first resultant magnetic field Z from Z=(VL-VR)/(VL+VR), based on the detected magnetic field strengths VR and VL (step 16-6). The first resultant magnetic field Z is either the resultant magnetic field Za of the right offset portion or the resultant magnetic field Zb of the left offset portion.
After computation of the first resultant magnetic field Z, the CPU 15-2 checks the value of flag f1 (step 16-7). If f1=0 does not exist, then it is judged that a vehicle is positioned at the right offset portion (a-point in FIG. 14). Then, the CPU 15-2 makes the first resultant magnetic field Z equal to Za (e.g., Za=Z) (step 16-8) and sets flag f1 to 0 (step 16-9).
When, on the other hand, flag f1 is 0, it is judged that a vehicle is positioned at the left offset portion (b-point in FIG. 14). Then, the CPU 15-2 makes the first resultant magnetic field Z equal to Zb (e.g., Zb=Z) (step 16-10) and sets flag f1 to 1 (step 16-11). Thus, each time a vehicle travels by distance L, the value of flag f1 is alternately set to 0 and 1 and the resultant magnetic fields Za and Zb are computed.
Note that during an initial travel of a vehicle, only either the resultant magnetic field Za or Zb is computed even when the vehicle travels by distance L. In this case, at the time step 16-9 has ended, the computation operation in the CPU 15-2 will return to step 16-1 again. Then, after computation of the resultant magnetic fields Za and Zb, the CPU 15-2 computes the second resultant magnetic field Zc=Za+Zb from the computed magnetic fields Za and Zb (step 16-12). Next, the CPU 15-2 accesses the memory 15-4 to read out the lateral displacement quantity X corresponding to the second resultant magnetic field Zc from the map stored on the memory 15-4 (step 16-13). With this, the lateral displacement quantity X of a vehicle can be detected within a detection range of W+D.
In addition, the detection range has been enlarged from conventional W to W+D, so it becomes possible to control a vehicle at a speed faster than a conventional speed even on a road where in the conventional apparatus the speed of the vehicle had to be reduced so that the vehicle does not travel out of the detection range.
The conventional apparatus, as previously described, detects the magnetic field formed on a road by the vehicle-mounted magnetic field sensors and, based on the result of the detection, recognizes the self-position of the vehicle. However, when a vehicle is displaced laterally or when substance or a magnetic body is disturbing the magnetic field formed on a road, the magnetic field is disturbed and the relation between the resultant magnetic field and the lateral displacement quantity, previously stored on the memory, is no longer established. Consequently, there arises the problem that a mistaken lateral displacement quantity is computed and causes a mistake in steering control.